[GH-277] Remove duplicate SQL parts in query generation

This issue is created automatically through a Github pull request on behalf of gigablah:

Url: https://github.com/doctrine/dbal/pull/277

Message:

Presently, it's possible to do this using QueryBuilder:

```php
$qb->select('u.*')
->from('users', 'u')
->addOrderBy('u.name', 'ASC')
->addOrderBy('u.name', 'ASC'); // duplicate
```
Which produces:
```sql
SELECT u.* FROM users u ORDER BY u.name ASC, u.name ASC
```

This patch removes duplicates from the `SELECT`, `SET`, `ORDER BY` and `GROUP BY` parts during SQL generation (there's already logic that removes duplicate `FROM` parts).

I've added test cases for each affected keyword as well.